A large number of welding operations are performed by the so-called "two pass" method. This is the most economical submerged arc welding method to join plates in a thickness range of 3 to 40 mm. The weld preparation is very simple, and in most cases a flame cut is sufficient. In the upper thickness range of approximately 16 to 25 mm, welding is generally carried out by the so-called "Y-weld", which facilitates adapting the welding data to the penetration depth of the arc. A third possible weld, particularly intended for materials with high plate thickness, is the double-Y-weld. The best known and economically most important uses of this third type of submerged arc welding is in ship building and in the manufacture of large pipes for transporting gases and liquids. Over the last few years, particular importance has been attached to the manufacture of large pipes for transporting natural gas and petroleum.
Submerged arc welded pipes are now being laid in regions subjected to particularly low temperatures in winter. Consequently, the pipe material and the welds have high toughness requirements at such low temperatures. Such toughness requirements are generally expressed in notch impact values, determined with Charpy V-notched specimens at temperatures between -20 and -60.degree. C.
Only since the 1970's have single-pass and two-pass welds been used in the manufacture of such pipes to produce the necessary quality. Previously, such welds had a microstructure much too coarse to obtain good notch impact values.
The discovery of the action of microalloys, particularly the action mechanism of small contents of Ti and B in combination with minimum contents of oxygen and nitrogen, permitted production of large single-pass and two-pass welds with very high toughness values at low temperatures. During the solidification and cooling process of the weld, these microalloying elements form a high proportion of acicular ferrite, instead of the coarse and brittle proeutectoid ferrite. Acicular ferrite is responsible for and a prerequisite for high toughness levels at low temperatures.
A very large number of publications have appeared over the last 13 years in this very important field of welding metallurgy. The International Institute of Welding (IIW) has also carried out extensive research in connection with this phenomenon. Such welds now form part of the prior art, e.g., Swiss Patent Application No. 9592/80.
The alloying of titanium and boron into the submerged arc welds is carried out with different methods, as follows:
(a) alloying the solid wire or wires using a welding powder giving low oxygen and nitrogen values;
(b) alloying a filler wire which is then used in the same way as a solid wire for submerged arc welding; or
(c) using a welding powder which alloys all or part of the necessary microalloying elements into the weld metal during the welding process, while using a standard, commercially available welding wire without microalloying elements.
Applicant's assignee has been producing filler wires according to method (b) for a number of years. Applicants consider this method to be the most accurate method for supplying the very small contents of microalloying elements in a metallurgically effective manner to the welding process (see Swiss Patent Application No. 9592/80).
The use of multiple wire welding is increasing in the manufacture of pipelines according to the spiral or longitudinal weld system. In multiple wire welding, a plurality of identical wires or electrodes are arranged in single file during submerged arc welding. Although each individual welding wire generally produces its own arc, the weld pools of all the participating wires combine into a single pool. The multiple wire arrangement permits use of plural thinner wires, in lieu of a single thicker wire, thereby facilitating higher welding speeds. According to the rule of thumb, two wires double the possible welding speed, three wires triple it, and four wires quadruple it. The use of more than four wires is not usual within the present state of the art.
According to the presently conventional methods, one to four identical microalloyed filler wires or solid wires are used for welding single and two-pass welds. These conventional methods are relatively expensive because microalloyed filler or solid wires are much more expensive than standard submerged arc welding wires. However, due to the much higher quality required for the welds, the higher priced microalloyed wires have to be used.